The development of an advanced clean-coal technology will permit the use of coal to replace oil and gas while eliminating the environmental penalty now associated with sulfur-containing coal. One way of producing fuel and power from coal is by way of pulverized coal combustion. Another, more efficient, way of doing so is by gasification of coal. Coal gasification produces a gas stream suitable for the production of electrical power, gaseous and liquid fuels, or other products. Before this can happen on a large scale, a low-cost, clean coal gas must be produced.
Coal gasification plants carry the promise of highly efficient utilization of coal. Electrical energy, for example, can be generated by the partial oxidation of coal in a gasifier/molten carbonate fuel cell system (MCFC) or in an integrated gasification combined cycle (IGCC) plant. An IGCC plant generates power by the direct contact of hot coal-derived gases with turbine blades, and is one of the most promising new technologies for the production of base-load electric power from coal.
While coal is the most abundant energy resource in the United States, and IGCC and MCFC plants have good generating efficiencies, current coal-based power generation imposes greater environmental burdens than oil or natural gas. Coal-derived gases contain particulates, tars, ammonia, alkali metals and sulfur. These materials are not only pollutants, but can cause corrosion, erosion or deposition on the turbine blades of a power plant.
Coal-derived gases contain significant levels of sulfur contamination. When coal is gasified, most of the total sulfur content is converted to hydrogen sulfide (H.sub.2 S). The hydrogen sulfide concentration in the coal gas depends on the amount of sulfur initially present in the coal and on the nature of the coal gasification process used. Gas-phase concentrations of hydrogen sulfide in the order of several thousand parts per million (ppm) are typical, and 10,000 to 30,000 ppm is not unusual. Sulfur contamination of the coal gas is an environmental problem and also an operational problem. Because sulfur is a useful chemical, its recovery is also worthwhile economically.
If not removed from the hot gasifier coal, the hydrogen sulfide would attack turbine blades in an IGCC plant, electrodes in fuel cells, and catalyst in synthetic fuels production.
In the environment, the un-removed sulfur species present in coal gases can react with oxygen and atmospheric water vapor to produce sulfuric acid and can contribute to the problems of "acid rain." The United States Environmental Protection Agency standards of Oct. 1, 1985 (40 C.F.R. Part 60, Subpart LLL) limit natural gas processing plants and petroleum refineries to sulfur emissions in the order of less than 90 parts per million (ppm), requiring sulfur removal in the range of 99% efficiency.
The European Community will require a minimum of 98.5% sulfur recovery rates by 1992, and the Federal Republic of Germany's regulations currently require up to 99.5% recovery. The United States New Source Performance Standards require at least 90% removal of sulfur for most new plants. IGCC plants, however, have the promise of greater than 90% removal of sulfur, and would largely eliminate the environmental penalties of coal use.
Economically, sulfur is valuable as a constituent of sulfuric acid, the largest single chemical consumed in the United States (over 11 million long tons of sulfur consumed in 1988). In the United States, elemental sulfur is typically recovered by steam injection from underground deposits, but this is thermally inefficient. Natural gas and petroleum processing is another large source, but these show signs of decline in the United States. Accordingly, if elemental sulfur were to be recovered as a by-product of the desulfurization of coal gas, the recovered sulfur would have a ready market.
To recover sulfur from the coal gas stream and to minimize the emission of sulfur compounds, an IGCC plant typically operates with a reaction step and a separate sulfur removal step. During the reaction step, coal is converted to product gas (synthesis gas, or "syngas") at high temperature. During the sulfur removal step, physical solvents are generally used to remove sulfur products and other contaminants from the crude syngas.
In a typical, "cold gas" sulfur removal process, the removal step reaction cannot take place at the high temperatures encountered in the reaction step. Thus, a cold gas approach requires (a) cooling of the hot (500.degree.-800.degree. C.) syngas to the relatively lower temperatures commonly needed for physical solvents, and (b) subsequent reheating of the cleaned syngas prior to its introduction into the gas turbine. These cooling and heating phases tend to increase capital costs and operating costs.
An IGCC plant has the potential for higher conversion efficiency, lower capital costs, and lower pollution impacts than pulverized coal-fired combustion even when used with cold gas cleanup systems. For economically more viable conversion of coal to gas without significant loss of thermal energy, there is a need for a "hot gas" cleanup system, capable of removing sulfur from the coal gas stream at high temperatures, in the range of 500.degree.-800.degree. C.
The use of hot gas cleanup can reduce capital costs and improve overall conversion cycle efficiency by eliminating the need to cool and reheat the gasifier outlet gases. It can also reduce wastewater disposal costs. Other coal gasification technologies besides IGCC and MCFC applications which would significantly benefit from hot gas cleanup include gasifier/diesel engine combinations, and processes for producing synthetic fuels from coal.
Many commercial processes are available for cold gas cleanup, but advanced hot gas cleanup systems are just now being tested at the pilot scale. Over the past decade, the United States Department of Energy and its Morgantown Energy Technology Center have made extensive efforts to develop high temperature regenerable desulfuring agents. Successful sorbents should absorb sulfur so as to provide efficient desulfurization, and should be long-lived or regenerable.
Current hot gas sulfur removal research is focused on regenerable, metal oxide sorbents that remove sulfur from the coal gas, and are then regenerated with air. Some of the metal oxides which have been tried include zinc ferrite, copper zinc oxide, and cuprous oxide.
The most developed candidate is zinc ferrite (ZnFe.sub.2 O.sub.4), which reacts as follows with the hydrogen sulfide (H.sub.2 S) contaminant of coal-derived gases to form zinc and iron sulfides (ZnS and FeS): EQU AnFe.sub.2 O.sub.4 +3H.sub.2 S+H.sub.2 .fwdarw.ZnS+2FeS+4H.sub.2 O.
The zinc and iron sulfide products of the absorption of hydrogen sulfide, when reacted with air, will regenerate the zinc ferrite starting material producing a sulfur dioxide (SO.sub.2) byproduct: EQU ZnS+2 FeS+5 O.sub.2 .fwdarw.ZnFe.sub.2 O.sub.4 +3 SO.sub.2
As can be seen, the regeneration process produces sulfur dioxide, which is a contaminant which must then be disposed of itself. The standard recovery method is to react the sulfur dioxide with limestone, producing ash. This process incurs significant costs for the purchase of limestone (ranging from $7.00 to $30.00 per ton) and for the disposal of the ash (ranging from $4.50 to $15.00 per ton). Disposal costs may be expected to rise as the number of available landfill sites is reduced. Another type of sulfur dioxide recovery method (known as the Direct Sulfur Recovery Processes or DSRP) reacts the sulfur dioxide with carbon monoxide (CO) and hydrogen gas (H.sub.2) in a sidestream of hot coal gases to produce water and elemental sulfur. The DSRP method incurs costs because the use of the coal gases as a reducing agent decreases the overall energy available from the gasifier by about 4%.
Current hot gas cleanup technology, involving regenerable zinc ferrite (ZnFe.sub.2 O.sub.4) and follow-on removal of the sulfur dioxide by-product with limestone or DSRP methods can cost approximately $425.00 per ton of sulfur. This amounts to about 5.7 mills per kilowatt hour, or as much as 9% of the busbar (ideal rated capacity) electrical cost of an IGCC. Clearly, an improved method of hot gas recovery could significantly reduce the cost of the electricity from an IGCC.
Full realization of the tremendous commercial potential of coal gas fueled power plants and related technologies awaits the development of an inexpensive and reliable hot gas clean up method for the removal of sulfur contaminants from the coal derived gas stream.
The successful sorbent must, therefore, be able to remove sulfur so as to leave sulfur levels in the gas stream of 20 ppm or less (a recovery rate greater than 99.8%); and it must also have physical and chemical stability in gas atmospheres of 500.degree. C. and above. A sorbent pellet will be reused in successive absorption cycles. Accordingly, and for the sake of economic efficiency, the pellet must be long-lived or, if short-lived, must be easily refabricated.
In addition to its chemical characteristics, the sorbent's physical characteristics affect its suitability for use in high temperature desulfurization. Among the relevant characteristics are durability, temperature stability, life span, and rate of utilization. Sorbent pellets are subject to physical and chemical degradation over successive process cycles: they may be broken by mechanical transport, fractured by multiple chemical reactions, and contaminated by gasifier ash which is not removed by upstream filtering.
Although zinc ferrite, and other metal oxides and mixed metal oxides have had some success in high temperature desulfurization of coal gases, they have limitations.
Thus, it can be seen that there is a need for an efficient high temperature desulfurization process that will remove as much as 99.8% of the hydrogen sulfide contaminants of the coal gasification stream. The desired process would use regenerable sorbents. The desired process would also consume unwanted by-products of the absorption/regeneration reactions so as to minimize the need for separate recovery and disposal of such by-products. The desired process would recover elemental sulfur in a useable form for resale.
Because the desired process would subject the sorbent pellets used in the system to conditions of heat, chemical reaction and pulverizing forces which tend to degrade the pellets, there is an additional need for a suitable pellet. If a long-lived pellet is not commercially feasible, the desired pellet must be one which is short-lived. The desired short-lived pellet must be capable of being refabricated. Accordingly, a method for the inexpensive recovery and reuse of the tin (or other metal species) from the degraded sorbent pellets is desirable. The desired method would involve the periodic removal of degraded pellets, the chemical recovery of the metal species from the degraded pellet, and the refabrication of the high surface area tin oxide (or other metal oxide) in a new pellet.